1. Field of the Invention
The present invention generally relates to hydrogen generation, purification, and recovery and, more particularly, to a method for preparing a porous glass membrane having selectivity for hydrogen while retaining permeability.
2. Background Description
Ceramic membranes are receiving considerable attention. Over the last ten years it has been demonstrated that membrane-based separation processes are commercially viable in a wide variety of applications. However, polymer-based membranes are used in the majority of these processes, and thus, there are inherent limitations in the operating temperatures and pressures that can be used. It is felt that ceramic-based membranes would offer significant improvements in the range of operating temperatures and pressures available. The question is how to make ceramic membranes with high selectivity and high permeability.
Several publications have appeared describing the use of porous glass tubes as separation media. The pore sizes that can be achieved are generally somewhat larger than molecular dimensions, thus, separation is based on Knudsen diffusion. This is not a very efficient separation mechanism. In an attempt to improve the performance of these porous glasses, a number of investigators have tried to deposit various types of materials in the pores under controlled conditions. The idea is to reduce the average pore size so that only hydrogen can pass through. While some success has been achieved, it usually comes at the expense of reduced permeability.
In recent years, increasing attention has been paid to global warming as a result of the release of greenhouse gases. The methane dry-reforming reaction (1)
CH4+CO2=2CO+2H2xcex94Hxc2x0298=247 kJ molxe2x88x921xe2x80x83xe2x80x83(1)
provides a pathway to convert carbon dioxide, a problematic greenhouse gas, and methane, a plentiful natural resource, into syngas (a mixture of CO+H2). Syngas is an industrially important feedstock that can be commercially transformed into ethylene glycol, MTBE, acetic acid, oxo alcohols, diesel, ethylene and several other important chemicals.
Fischer and Tropsch (Die zusammensetzung der bei der erdxc3x6lsynthese erhalten produkte, Brennstoff Chem. 2(9)(1928)21) were the first to propose the dry-reforming reaction for methane conversion to syngas. In recent years, many researchers have explored this route towards syngas production. The studies have concentrated mainly on noble metals and Ni catalysts on various supports, and the results have been mixed, with some reports of catalysts being active and stable for long periods of operation, while others of catalyst undergoing coking and deactivation. A consensus from these studies is that noble metal catalysts are usually resistant to coking (with Pt being an exception). For Ni catalysts however, only low loading catalysts had good activity without appreciable coking while high loading ( greater than 10% metal) catalysts usually deactivated due to coking. Wang, et al. (Carbon dioxide reforming of methane to produce synthesis gas over metal-supported catalysts: State of the Art, Energy and Fuels 10(1996) 896) have provided a comprehensive summary of many of the catalysts used in the carbon dioxide reforming of methane.
The conversion of methane in the fixed-bed mode of operation is limited by the reversibility of the reforming reaction. For such reversible reactions, preferential removal of one or more of the products during reaction will cause a shift in equilibrium, thereby overcoming thermodynamic limitations. Membranes can bring about such selective removal of species during reaction and hence reactors incorporating such membranes have been used to increase reaction yields. Membranes have also been used in applications where controlled introduction of reactant(s) is necessary to reduce hot spots in a catalyst bed or to avoid undesirable side reactions. Reactors incorporating membranes offer advantages over conventional fixed-bed reactors that include higher energy efficiency, lower capital and operating costs, compact modular construction, low maintenance cost, and ease of scale-up.
Some of the earliest studies on membrane reactor applications used noble metal membranes for several hydrogenation and dehydrogenation reactions, and high conversions together with good selectivity were reported. Use of a silver membrane in the oxidation of ethanol resulted in a 50% improvement over equilibrium. More recently, considerable work has been done with ceramic membranes. H2S decomposition studies have been conducted in a porous-glass membrane reactor resulting in selective separation of H2 from the reacting mixture and conversions twice as high as equilibrium were reported. The dehydrogenation of cyclohexane in reactors using platinum impregnated Vycor (Vycor is a porous glass which is essentially borosilicate glass with boron removed, and is commercially available from a variety of sources including Corning), palladium, and porous glass membranes resulted in conversions 2.5 to 5 times higher than equilibrium conversion. In studies on the oxidative dehydrogenation of ethane and the dehydrocyclodimerization of propane using alumina and palladium-silver membranes respectively, results indicated no improvement in conversion but there was improved selectivity to products. The dehydrogenation of methanol and n-butane in alumina membrane reactors was studied with 50% improvement in conversions obtained in the membrane mode of operation as compared to the fixed-bed mode of operation. The methane steam reforming reaction in metal dispersed alumina membrane reactors has resulted in conversions twice as high as equilibrium. The same reaction in an alumina membrane reactor has provided conversions 20% higher than the equilibrium level.
Several studies have focused on the development of selective membranes that provide high selectivity by suitably modifying a porous ceramic support. Sol-gel processing and CVD have been the methods of choice by most researchers. Sol-gel modification provides good selectivity and permeability as opposed to CVD methods where there is an accompanying loss of permeability, though the selectivity is enhanced. The sol-gel method however, suffers from a lack of reproducibility.
The silica modified membranes developed by several researchers suffer from loss of permeability (as much as 50% or greater in the first 12 h) on exposure to moisture. This has been attributed to the removal of Sixe2x80x94OH groups leading to the formation of Sixe2x80x94Oxe2x80x94Si bonds which close pore channels. This phenomenon is termed as densification. Moisture apparently catalyzes this reaction particularly at higher temperatures. Densification not only leads to lower permeability but also causes embrittlement of the silica film that compromises selectivity.
It is therefore an object of the invention to provide a membrane which is extremely selective to hydrogen which also maintains good permeability.
It is another object of the invention to provide an inorganic ceramic composition, which is stable at high temperatures, in the presence of steam, and under pressure.
According to the invention, a membrane, called Nanosil throughout this application, is formed by chemical vapor deposition (CVD) of tetraethyl orthosilicate (TEOS) at high temperature in the absence of oxygen or steam. This membrane has selectivities of 100% with respect to CH4, CO, CO2 and H2O. The invention can be practiced with other silica precursors such as tetraethyl silicates, tetra isopropyl silicates, chloro-, dichloro-, and trichloromethylsilanes, and other silicon compounds. An important feature of the invention is that the silica precursor be decomposed in an inert atmosphere (lacking oxygen or steam). Decomposition can be accomplished by high temperature exposure, laser exposure, or other means.